This invention relates to a process for producing a precision metal part, such as a part of a watch, of a sintered body having a high density near the theoretical density.
Recently, various sintered parts using metal powder or ceramic powder have been developed and widely utilized in the fields of general industrial materials, precision machine parts, electronic parts, electric parts, motor car parts and the like. According to this development, the dimensional accuracy, properties, forms etc. required for the parts are severe. A usual powder for molding is produced by using a spray dryer and molded by rubber pressing to obtain a molded body for sintering. The above processes are very complex and the yield of the molded body is very low. Moreover, molded bodies having a complex form cannot be produced.
In order to meet these requirements and problems, some methods were developed comprising imparting plasticity to metal powder or ceramic powder by adding a suitable resin, molding it by the injection molding process, removing the resin in the molded body through thermal decomposition and then sintering to obtain a desired metal or ceramic powder injection molded part (e.g., Japanese Patent KOKOKU No. 51-29170, Japanese KOKAI Nos. 55-113510, 55-113511, etc.). The above methods are noted as removing the organic binder, a resin from the molded body in a short period without inducing cracks, blisters and deformations, as well as not generating cracks in the molded body during the injection molding. However, even though the above problems are resolved, it is difficult to produce precision metal parts having a density near the theoretical density by powder molding, which is an object of the invention. Besides, in general, the oxygen content of metal powder increases by rendering the mean particle size smaller. When the metal powder is reduced in order to decrease the oxygen content, the mean particle size tends to increase due to the condensation or the agglomeration of particles. In the conventional methods, the purity of metal powder is regarded as important and low oxygen content metal powder is used, which is not spherical but irregular shape.
Incidentally, most parts of conventional watches were made of metal and produced by hammering, such as pressing, or cutting, such as lathe processing. However, these processes require a processing time and after steps such as deburring, which increase the labor time. Recently, engineering plastic materials have been developed and many parts of watches are now produced by injection molding.
The conventional gear illustrated in FIG. 6 is a minute wheel 21 which transmits the rotation of the center wheel, as well as moving the minute hand, to the hour wheel moving the watch with deceleration, and consists of two parts, i.e. a minute gear 22 and a minute pinion 23. The minute gear 22 has a hollow disc shape with teeth for engaging the pinion of the central wheel on its circumference, and the minute pinion 23 has teeth on its circumference for engaging the teeth of the hour wheel on the reverse side to the above portion engaging the minute gear 22. The conventional minute gear is produced according to the flow diagram shown in FIG. 7. In the figure, the left side flow scheme indicates the process of producing the minute gear and the right side flow scheme indicates the process of producing the minute pinion. In the process of producing the minute gear, a strip material made of brass (hereinafter referred to as BS) (step A) is punched into a disc (step B). Several pieces of the disc are superposed and milled to form a gear (C). On the other hand, in the process of producing the minute pinion, an iron bar material (step D) is pressed (step E), carburized (step F), hardened (step G) and then tempered (step H). The bar material is cut into a desired form (step I). Subsequently, barrel glazing (step J) and plating (step K) are conducted to complete the minute pinion 23. The minute pinion 23 is incorporated into the minute gear (step L) to complete the minute wheel 21. As mentioned previously, the above conventional process has various problems, such as many processing steps requiring a long time and many processing machines. Moreover, the assembling of plural parts is also necessary which causes assembly troubles. Accordingly, producing more complex part or a smaller part of a watch is difficult by the above manufacturing process.